1. Field of the Invention
The present invention relates to a thiophene derivative and an organic electroluminescent (hereinafter it is simply referred to as “EL”) element. Particularly, it relates to a novel thiophene derivative which is useful for producing an organic electroluminescent element, a polymer thereof, and an organic EL element.
2. Description of the Related Art
An organic EL element comprises a couple of electrodes facing each other and an organic light emitting layer disposed between the couple of electrodes, wherein the organic light emitting layer emits light when an electron injected from one electrode and a positive hole injected from another electrode recombine in the organic light emitting layer. Research and development of such an element fully started when M. Pope, H. P. Kallmann et al. found light emission of a single crystal of anthracene generated by impressing direct voltage in 1963. In 1987, an organic EL element utilizing a laminated structure of organic thin layers was announced for the first time by T. W. Tang, et al. of KODAK.
Thereafter, the organic EL element has been researched and developed aiming for improvement in functions from various aspects such as material, a layer constitution, a method for constituting layers, a method for producing elements or the like. FIG. 1 shows a layer constitution of a general organic EL element. As a material for forming an organic positive hole transporting layer, which has large influence on light emitting property and color property among the layers, polybenzo[c]thiophene sulfonic acid is proposed (Japanese Patent Application Laid-Open (JP-A) No. Sho. 61-17581). Polybenzo[c]thiophene sulfonic acid is a useful conductive polymer material, however, polybenzo[c]thiophene sulfonic acid is basically synthesized by sulfonation of polybenzo[c]thiophene in the presence of sulfuric acid and it is significantly difficult to introduce a sulfonic group to all polybenzo[c]thiophene units. In the case of synthesizing a copolymer of benzo[c]thiophene sulfonic acid and other monomer in order to exhibit various functions, sulfonation is performed after the copolymerizing process, thus, the monomer structure to be copolymerized with polybenzo[c]thiophene may be unnecessarily affected by the sulfonation, which is not efficient.
As aforementioned, various organic materials have been developed to form each layer. Many organic EL elements comprise an organic positive hole transporting layer and an organic light emitting layer as essential layers. Since a material constituting the organic positive hole transporting layer is generally hydrophilic, the material for forming the organic positive hole transporting layer is dissolved or dispersed in water to obtain a coating liquid and the coating liquid is applied on a substrate to form the organic positive hole transporting layer. On a surface of the organic positive hole transporting layer, an organic light emitting layer is formed. Since a material for forming the organic light emitting layer is generally lipophilic, the material for forming the organic light emitting layer is dissolved or dispersed in an organic solvent to obtain a coating liquid and the coating liquid is applied on the organic positive hole transporting layer to form the organic light emitting layer.
Generally, an organic El element is easily affected by moisture, and a performance of element is easily deteriorated by moisture. Hence, in many cases, it has been attempt to eliminate adverse affect of moisture by sealing the organic EL element sufficiently. However, as aforementioned, since the organic positive hole transporting layer is formed with the coating liquid containing water and the material for forming the organic positive hole transporting layer is a hydrophilic material, it is significantly difficult to completely remove even a minute amount of moisture from the organic positive hole transporting layer. In order to moderate adverse influence of the minute amount of remained moisture, efforts such as inclusion of an absorbent in an organic El element upon sealing or the like have been made. However, there are problems that such an effort is cumbersome and requires high production costs.